Well... I don't really see the point of downloading the whole site (...) a program that would download every book for an author or a list would make much more sense than strictly downloading the whole website

Main point in downloading the whole website is: if it gets offline for some reason (eg.: you decide to stop the project or you have copyright judicial issues) people would still have the whole collection in their machines. Maybe you should periodically create several DVDs that people could download from the website and from P2P networks. You could also make such DVDs for given authors or special hits.